There are several drilling techniques which may be employed for sampling underground deposits, e.g. sands and gravels. The particular choice of technique is governed by the ground conditions, the technical objectives and a foreknowledge of the water table conditions. In hard rock, the diamond coring technique is used, and the competence of the strata usually prevents contamination of samples. However, in soft rock, e.g. superficial sands and sand-rock, the strata are likely to collapse into the bore hole, particularly under wet conditions. In extreme cases this phenomenon is known as "running sand".
One drilling technique which has been used for soft strata employs continuous flight augers which involves the sinking of an unprotected hole, with the augers pumping material from the strata below to the surface. Whilst this drilling technique has the advantage of speed, contamination is likely to occur beneath the regional water table or beneath perched water tables. Accordingly, no great confidence can be placed in the samples by this technique.
An alternative technique is "shell and auger" drilling using a percussive method. In this technique the bore hole is advanced by casing, with material from within the casing being delivered to the surface by a cutting cylinder attached to a cable and winch. The technique is usually very slow and in cases of running sand may fail completely. However, the advantage of the technique is that under many conditions greater confidence can be placed in the samples derived from below water tables, as the bore hole is sealed off from in-flow of material from levels above the cutting area.
A further technique which may be used for sampling underground deposits employs a duo-pipe system. This system combines the flushing mechanisms usually associated with diamond coring with the principle of casing the bore hole. The system comprises two concentric tubes attached to the boring head arranged in such a way as to allow fluid under pressure to pass down the annular passage between the tubes whilst they are rotated together during the drilling operation. A series of galleries is provided at the boring head which allows the fluid under pressure to escape into the inner tube in such a manner that any particles of cut material in the region of the galleries are entrained in the fluid flow and transported to the surface through the inner tube for collection and analysis. A small proportion of the fluid under pressure may be circulated to the base of the boring head for cooling and dissipates up the outside of the tubes creating a useful pressurised environment countering any tendancy for downward flow in this area.
Heretofore, the boring heads which have been employed in such duo-pipe systems have generally been provided with an inlet for cut material which is coaxial with the axis of rotation of the boring head. The cutting teeth of the boring head are generally disposed around this material inlet. The fluid supply galleries in boring head are normally directed at right angles to the passage for material return or are directed towards the cutting face relying upon a back pressure building up against the cutting face to divert the fluid up the return pipe. It has been found that this arrangement is not always efficient in flushing cut material to the surface for analysis, particularly when the boring head penetrates clay or gravel and this inefficiency may result in contamination of the samples collected.